A stein is a large mug most often used for drinking beer. A staple for German celebrations for hundreds of years, German beer steins have gained popularity around the world, with many people referring to them as beer steins. Steins often include an attached, matching, hinged lid with a thumb lever for easy opening, but there's no rule for these mammoth mugs to have a lid to be considered a stein.
Though the stein is most closely associated with German cultures, this style of mug wasn't exclusive to Germany alone. England has historically used similar glassware, called tankards, to hold their beer.
The stein's semi-formal but unofficial capacity is two pints, and many contain a waterline etched on the inside to indicate the appropriate amount to pour.